Various types of driving devices which use an electromechanical conversion element such as a piezoelectric element have been proposed.
For example in the fixed element type of a driving device that is schematically shown in FIG. 14(a) to FIG. 14(c), one end in the extension and contraction direction of the piezoelectric element 92 which is the electromechanical conversion element is fixed to the fixing member 91 and the drive friction member 94 is fixed to the other end. The drive friction member 94 moves in the feed direction and the return direction based on the extension and contraction of the piezoelectric element 92. The moving member 93 engages with the drive friction member 94 due to frictional force.
By applying voltage to the piezoelectric element 92 and causing the velocity of the piezoelectric element 92 to be different at the time of extension and the time of contraction, the moving body 93 is driven. FIG. 14(a) to FIG. 14(c) show the states at Pa, Pb and Pc when voltage is applied in the operation waveform shown in FIG. 15.
In the section Pa-Pb in FIG. 15, when the voltage waveform rises gently, the piezoelectric element 92 expands relatively slowly and changes from the state in FIG. 14(a) to the state in FIG. 14(b). At this time, there is little or no sliding of the moving body 93 against the drive friction member 94, and the moving body 93 is moved substantially integrally with the drive friction member 94.
Next in the section Pb-Pc, when the voltage waveform descends rapidly, the piezoelectric element 92 contracts relatively quickly and the drive friction member 94 rapidly returns to the start position. At this time, sliding occurs between the drive friction member 94 and the moving body 93, and the moving body 93 remains substantially stationary and only the drive friction member 94 returns to the start position. As a result, as shown in FIG. 14(c), the moving body 93 moves in the feed direction from the start position in FIG. 14(a).
The moving body 93 moves along the drive friction member 94 due to this cycle being repeated.
It is to be noted that if the voltage with a return waveform which comprises a steep rise and a gentle fall is applied to the piezoelectric element, the moving body 93 moves in the return direction.
The methods for applying the voltage of the sawtooth waveform to the piezoelectric element 92 include the following two methods.
FIG. 16 shows the first method. As shown in FIG. 16(a), the 8-bit, 0-5V sawtooth waveform for example, is generated by the waveform generator 95 which is a digital analog converter, and this is input into the power amplifier 96, and the sawtooth waveform for driving that has been amplified to 0-10V for example is applied to the piezoelectric element Pv. The sawtooth waveform used for the feed waveform shown in FIG. 16(b) and the return waveform shown in FIG. 16(c) can be generated by adjusting the waveform generator 95.
FIG. 17 and FIG. 18 show the second method. As shown in FIG. 17, a circuit comprising constant current circuits 98a and 98b, and switch circuits 99a and 99b are used in order to apply the voltage of the power source 97 to the piezoelectric element Pv, and the feed waveform and the return waveform are generated by alternately operating the constant current circuits 98a and 98b and switch circuits 99a and 99b. 
More specifically, the feed waveform and the return waveform are generated by forming the digital circuit shown in FIG. 18(a) for example and inputting control signals such as those shown in FIG. 18(b) into terminals Ra-Rd.
That is to say, Hi signal is input to the terminal Ra, and after the voltage which applied to the piezoelectric element Pv via the constant current circuit 98a is gradually increased, Hi signal is input to the terminal Rb, and the piezoelectric element Pv is grounded via the switch circuit 99b, and the voltage that is applied to the piezoelectric element Pv is rapidly reduced, and the feed waveform Ha is generated.
In addition, Hi signal is input to the terminal Rc, and after the voltage from the power source 97 is applied to the piezoelectric element Pv via the switch current circuit 99a, Hi signal is input to the terminal Rd, and is grounded via the constant circuit 98b and the return waveform Hb is thereby formed.
However, because in the first method, the waveform generator 95 and the power amplifier 96 are needed, and in the second method the constant current circuits 98a and 98b and switch circuits 99a and 99b are needed, the circuits are complex and the cost is high.
As a result, a driving device having a simple circuit structure has been proposed (See Patent Document 1). In this driving device, drive control is performed using three voltage values (maximum value, minimum value, and middle value) as the voltages that are applied to the piezoelectric element.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-80964
In the driving device of the Patent Document 1, low velocity driving is possible to a certain extent using the maximum value, the minimum value and the middle value, but lower velocity driving required for performing servo-control and the like is difficult. In addition, the operation of smoothly changing the drive velocity from the feed direction (front direction) to the return direction (opposite direction) is difficult. Furthermore, no consideration has been given to reducing power consumption at the time of low velocity driving.